Add logic to login to Docker Hub
Add logic to login to Docker Hub

This pulls a Docker ID and access token from SSM Parameter store for a dedicated
Docker user and logs in as that user to Docker Hub. This works around the issue
where anonymous API access to Docker Hub from AWS CodeBuild IP addresses results
in rate limiting the docker pulls of the Docker Base Image (e.g. centos)

Fixes #446

mozilla-iam/sso-dashboard,mozilla-iam/sso-dashboard,mozilla-iam/sso-dashboard,mozilla-iam/sso-dashboard